Process for making stainless steel



Patented Dec. 17, 1946 PROCESS FOR MAKING STAINLESS STEEL Stephen F. Urban, Chicago, Ill., assignor to Illinois Steel Corporation, a corporation of New Jersey No Drawing. Application March 19, 1943, Serial No. 479,788

2 Claims.-

Individuals versed in the making, fabricating and use of stainless austenitic steels are fully aware of the proposition that normal carbon contents in these steels, namely, 0.05 to 0.15 percent, are in some instances objectionable, provided that the said carbon has been precipitated intergranularly by certain treatments such as occur during welding.

This intergranular carbide precipitation phenomenon becomes evident only when austenitic stainless steels are heated within a certain temperature range, and as mentioned before, this precipitation is related to the carbon content of the stainless austenitic steel, after which heating they are sensitive to intergranular corrosion or disintegration. Heating in the sensitive range of temperatures cannot be avoided in certain operations, such as in the Welding of austenitic stainless steels. The reason for this lies in the fact that during the welding operation, heat is applied in sufiicient quantity to cause fusion, thereby automatically establishing two thermal gradients in a direction away from the weld metal, which gradients are more or less symmetrical with respect to the weld proper. It therefore would be anticipated that a certain distance away from the weld, and on both sides of it, austenitic stainless steels are heated in the sensitive temperature range Where carbide precipitation occurs. The precise distances of these zones from the weld varies with welding operations, such as the type and speed of welding and the thickness of the steel being welded, but this is of no importance because these zones are always present and are very susceptible to intergranular corrosion when the fabricated articles of austenitic stainless steel are subsequently exposed to certain corrosive media, thereby resulting in localized attack at the forementioned zones.

The seriousness of this condition has been recognized by manufacturers of stainless steels and the condition is eliminated by the addition of such elements as titanium and columbium. The addition of these elements has, however, economic disadvantages in that these elements are very expensive; further, their presence multiplies certain fabricating problems such as hot rolling or seamless tube piercing. These special additions would not be necessary if stainless austemtic steels could be made commercially sufii- I 2 ciently low in carbon, and the basis of my invention is a process for the manufacture of such very. low carbon austenitic stainless steels.

In addition to minimizing or eliminating intergranular corrosion in austenitic steels, by incorporating in them such elements as titanium or columbium or reducing the carbon content to a sufficiently low value, for prevention of intergranular or local corrosion near the weld zone for example, it is at times desirable to increase general corrosion resistance away from the weld zone by incorporating copper into the composition. For the latter purposes copper is usually in the range 1 to 3 percent and is used in those austenitic stainless steels that are subjected to more corrosive conditions than similar composition without copper. For example, copper bearing austenitic stainless steels containing 16 to 20 percent chromium and 8 to 12 percent of nickel or manganese or combinations of nickel and manganese in the range 6 to 14 percent, are more resistant to mineral acid solutions than similar steels without copper. The eflect of copper is well described in prior art and technical literature.

From the foregoing it is evident that a copperbearing austenitic stainless steel has in certain instances advantages over non-copper bearing steel, but that copper is of no aid in preventing intergranular or local corrosion. Both conditions can be met by my invention which comprises a copper bearing austenitic stainless steel of less than 0.03 percent carbon, which carbon content is a. result of adding copper oxide rather than copper, to the steel during its manufacture.

The said carbon content yields a steel that is as insensitive to intergranular corrosion as steels containing titanium or columbium, which elements are commonly employed for insensitivity. This advantage is the essence of my invention; which is described below in detail.

I have found by experiment that an addition of copper oxide to a charge of steel scrap and nickel will cause the former to reach very much lower carbon contents, after melting, than are possible when copper is added in the elemental form. The procedure described yields a bath of about 0.015 to 0.025 percent carbon, which is deoxidized with sufliicient silicon or any other deoxidizer so that the oxygen content is below the 0 Mn 8 I P Si Cu Ni Cr where all values are in weight percent.

It will be noted here that the carbon content is less than 0.030 percent, which carbon content 01' less makes austenitic stainless steels as insen-- sitive to intergranular corrosion as the/usual copper bearing steel containing 0.05 to 0.15 percent carbon and proper amounts of titanium or columbium to impart immunity to the forementioned intergranular type of corrosion. Prevention of intergranular corrosion in copper bearing austenitic stainless steels by my invention, without the use of titanium or columbium has the advantage of economy and more specifically, the latter elements impart undesirable hot working characteristics.

In connection with the above and the claims to follow it is obvious to persons versed in the art that oxygen compounds of copper would be as efiective as copper oxide and for this reason the claims to follow are not to be limited to the oxide only.

I claim:

1. A process of manufacturing a copper-bearing austenitic stainless steel insensitive to intergranular corrosion, involving additions of an OK- ygen compound of copper to a charge of plain carbon steel scrap and nickel in an electric furnace, thereby causing a suflicient loss incarbon content, to provide a final carbon content, after subsequent additions of manganese, chromium and the like, of not more than 0.03 percent.

2 A process of manufacturing an austenitic stainless steel containing 1 to 3 percent of copper,

wherein the steel is rendered insensitive to intergranular corrosion by an addition of an oxygen compound of copper, to a charge of plain carbon ,steel scrap and nickel in an electric furnace,

thereby causing a suflicient loss in carbon content, that provides a final carbon content, after cent.

STEPHEN F. URBAN. 

